Speaker Power Detection Circuit

This is an interesting way to monitor your speakers. [Keith] put together this speaker line monitor after a commenter requested it on his blog. It’s designed to check for power on the speaker line and drive a logic/led output. Apparently there’s some risk of shorting your amp, so he’s planning to redesign the input stage. Still, it’s neat little hack to keep an eye on things. Personally, I’d just put em on their own class A amp and automate the power switching, but there are plenty of situations where this could be useful.

7 thoughts on “Speaker Power Detection Circuit

  1. @ #1) as far as I can figure, the only way to determine power handling for a speaker is to read the specs or blow it. I could be wrong, but I’m pretty sure there’s no way to electrically determine a speaker’s power handling.

  2. wouldn’t it make more sense to create a loop in the speaker wire around a metal core and check for induced current in the core? that way the detector circuit is completely separate and has no way of destroying your amp or speakers.

  3. @ #3:
    Or you could use one of those…what are they called…isolator transformers I think. Same basic idea, but it’s a pre-made part to do what you were saying…much easier.

  4. hebd:

    I thought about calling my project a “speakers are on detector,” but that sounded too goofy, so I settled for “speaker power detector.” I sympathize with your confusion, and I’ll cheerfully take suggestions for a more descriptive name.

    Regarding your desire for a device to adjust amplifier power to the capacity of a speaker, the best way to do that is probably your ears. :-) Seriously, assuming a clean amplifier at least as powerful as the speakers, in my experience, you’ll know when you’re reaching the limit of the speakers.

    I think the inductance of a speaker may change subtly as it’s driven closer to its maximum excursion, but that’s not something that strikes me as easy to measure automatically. There may be information on that in the Elliott Sound Products (http://sound.westhost.com/) articles or projects section, although I can’t find it at the moment.


    Two goals of the project, based on the resources of the fellow who requested it, were that it couldn’t involve modifying equipment, and that it had to be constructable out of components you can find locally in any town, e.g. at Radio Shack.

    Using the speaker wire’s inductance is a neat idea, and I honestly didn’t think of it; but I’m also not confident of my ability to design a reliable circuit for it that anyone could build out of Radio Shack parts. Maybe I’m overestimating the difficulty of detecting the induced current?

  5. I used to use a light bulb directly on the speaker terminal. The bulb I used did not have a filiment on it. It would light up when there was audio. The louder the sound the brighter the light.

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